1. Field of the Invention
The present invention relates to an image forming apparatus, and, more specifically, the invention relates to a method for cleaning toner contaminating an image carrier or an intermediate transfer unit.
2. Description of the Related Art
There are known image forming apparatuses, such as copy machines and printers, having a plurality of image forming units that emit light modified according to the data recorded on an image carrier (i.e., a photoreceptive drum) from a laser beam unit or a light emitting element such as an LED to form an electrostatic latent image and to develop the electrostatic latent image through an electrophotographic process and transfer this image onto a receptor such as transfer paper or an intermediate transfer belt.
Such an image forming apparatus forms a color image on a receptor such as transfer paper by disposing the transfer paper on a transfer belt and transferring an image for a first color onto the transfer paper by bringing the transfer paper close to or in contact with an image forming unit for the first color. Subsequently, an image for a second color is transferred over the first image on the transfer paper by bringing the transfer paper close to or in contact with an image forming unit for the second color. The same steps are repeated for transferring images for the third and fourth colors over the first and second images formed on the transfer paper.
There are other image forming apparatuses that transfer images for the first to fourth colors in sequence onto an intermediate transfer belt, which is also a receptor, and then transfers the image of the four colors simultaneously onto transfer paper to form a color image.
For such a known image forming apparatus, each color image formed on each photoreceptive drum is occasionally become out of register on the transfer paper or the intermediate transfer unit because by an error in the mechanical attachment of the photoreceptive drums, an error in the light path length of the laser beams, a change in the light path, or warping of the LED due to the ambient temperature.
Therefore, for a known image forming apparatus, pattern images for correcting the registration of the images of each color are formed on each photoreceptive drum and, then, are transferred to the transfer belt or intermediate transfer belt. By reading these pattern images by a CCD sensor, incorrect registration of each color image on the photoreceptive drums is detected. According to the detected results, change in the light path length or change in the light path are compensated for by electrically correcting the image signals or by moving the mirror installed in the midst of the light path of the laser beam.
To improve the stability of the color and density of the overlaid color images formed by a known image forming apparatus, pattern images (patch images) printed with a toner having a predetermined density are formed on the intermediate transfer belt and are read by a detecting unit to determine whether or not the density of a images are within a predetermined limit. When the density greatly exceeds the limit, the density of toner included in a developer and/or the bias voltage applied to a primary charged unit is adjusted.
Japanese Patent Laid-Open No. 2002-62709 discloses a transfer belt cleaning unit and an intermediate transfer belt cleaning unit for efficiently removing pattern images, which are used for the registration and/or for stabilizing the density of toner, formed on a transfer belt or an intermediate transfer belt.
Furthermore, Japanese Patent Laid-Open No. 2001-305873 discloses a cleaning unit for efficiently cleaning the pattern images for adjusting the registration or for stabilizing the density of the toner, wherein the toner of the pattern images on the intermediate transfer belt are removed after the toner is electrically neutralized by a neutralizing unit.
Unfortunately, for such a known image forming apparatus, the toner of the pattern image formed on the intermediate transfer belt may contaminate units in contact with and/or in the vicinity of the intermediate transfer belt. Moreover, for an image forming apparatus that forms a toner image on an intermediate transfer belt, the toner of the image might contaminate units in contact with and/or in the vicinity of the intermediate transfer belt when the operation of the image forming apparatus is resumed after image formation is interrupted due to unsuccessful delivery of a recording material.
When an image is formed after the toner contaminates a unit, the back side or the edges of the recording material may be smeared with the toner. In such a case, the unit soiled with toner must be removed and cleaned.
For an image forming apparatus having a plurality of photoreceptive drums 1a, 1b, 1c, and 1d, as illustrated in FIG. 8 or 9, a secondary transfer roller 57 or 82, which is always in contact with an intermediate transfer belt 51 and transfers a toner image on the intermediate transfer belt onto a recording material, may be contaminated by the toner. In such a case, the secondary transfer roller 57 or 82 has to be removed and cleaned. To remove and clean the secondary transfer roller 57 or 82 is not preferable because of the negative impact caused by the removal and the decrease in productivity.
For an image forming apparatus having a transfer fixing unit 80 with a heat source, as illustrated in FIG. 9, the operation of the apparatus may be interrupted due to failure of delivering pattern images or a recording material. When the operation of the apparatus is resumed, the residual toner on the intermediate transfer belt passes the vicinity of the heat source and melts on the intermediate transfer belt. The molten toner adheres to the intermediate transfer belt and becomes difficult to remove it from the intermediate transfer belt.